Varied morphology carbon nanotubes

ABSTRACT

The present invention describes the preparation of carbon nanotubes of varied morphology, catalyst materials for their synthesis. The present invention also describes reactor apparatus and methods of optimizing and controlling process parameters for the manufacture carbon nanotubes with pre-determined morphologies in relatively high purity and in high yields. In particular, the present invention provides methods for the preparation of non-aligned carbon nanotubes with controllable morphologies, catalyst materials and methods for their manufacture.

RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.10/151,382, filed May 20, 2002, which claims priority to U.S.Provisional Application Ser. No. 60/292,486, filed on May 21, 2001, andthe entirety of these applications are hereby incorporated herein byreference for the teachings therein.

STATEMENT AS TO FEDERALLY SPONSORED RESEARCH

The present invention was made with partial support from The US ArmyNatick Soldier Systems Center (DAAD, Grant Number 16-00-C-9227),Department of Energy (Grant Number DE-FG02-00ER45805) and The NationalScience Foundation (Grant Number DMR-9996289)

FIELD OF THE INVENTION

The present invention relates generally to carbon nanotubes of variedmorphology, catalyst materials for their synthesis, and apparatus andmethods for controllably manufacturing carbon nanotubes withpre-determined morphologies. More particularly, the present inventionconcerns non-aligned carbon nanotubes with controllable morphologies,catalyst materials and methods for their manufacture.

BACKGROUND OF THE INVENTION

Carbon nanotubes (CNTs) offer significant advantages over othermaterials in that they possess substantially higher strength-to-weightratio and superior mechanical properties. A major limitation to theirlarge-scale commercialization however, has remained the need for largequantity, cost-effective production methods. Conventional syntheticmethods for synthesis of CNTs utilize arc discharge, laser ablation andchemical vapor deposition (CVD). Existing manufacturing methods usingCVD are mainly directed toward obtaining aligned monolayer arrays ofCNTs on a catalyst surface that is comprised of either a metallicsubstrate, or a non-metallic substrate whose surface is coated with ametallic material.

Metal catalysts for CNT synthesis disclosed in the art involve thedeposition of a transition metal catalyst layer as a coating on asubstrate by standard methods such as metal vapor deposition andmagnetron sputtering. Such methods involve a combination of metallic(non-catalytic) and non-metallic substrates coated with a surface layerof a catalytic metal such as iron. They however, require relativelyexpensive and complex reactor apparatus, and typically require a highvacuum (10⁻⁵ to 10⁻⁷ torr) environment. Furthermore, such methods areonly capable of providing a uniform flat surface layer of the metalcatalyst on the substrate on which CNT formation and growth can occur.The surface area of the catalytic metal layer therefore, issubstantially similar to that of the substrate on which it is deposited.Since CNT yield is directly related to surface area of the catalyticsurface, substantially large areas of metal coated substrate is requiredto synthesize large quantities of CNTs, which is impractical in terms ofexisting limitations of the reaction apparatus.

A mesoporous silica sol-gel catalyst impregnated with iron was disclosedby Li et al. (Science, Vol. 274, (1996), 1701) for the synthesis ofaligned carbon nanotubes. The method described by Li requires thepreparation of large, flat surfaces of the iron impregnated mesoporoussilica substrates with uniform distribution of pores. According to Li etal., preparation of such large area catalytic substrate is hampered bythe inherent tendency to shrink, crack and shatter during theirpreparation. Meticulous drying procedures therefore, are required tomaintain the integrity of the catalyst to obtain large area surfaces,which is critical for obtaining high density monolayer CNT arrays.Imperfect catalyst preparation can severely limit yields of CNT product.Also, CNT synthesis by the process of Li et al. requires a reactiontemperature of 700° C., which is impractical for substrates such as flatpanel glass. Methods for producing an aligned array of linear CNTs on asubstrate surface has been described in WO 99/65821 by Ren et al. inplasma conditions under an applied electrical field. Such methodshowever, require high vacuum conditions, which is difficult to achievein large reactors in a commercially viable CNT manufacturing processes.

Although such methods are capable of providing highly pure, alignedCNTs, they are not best suited for large-scale production due to lowvolume (typically several milligrams to grams per batch), low yieldsbased on amount of catalyst and high manufacturing cost. Furthermore,existing methods do not allow control of nanotube morphology, tubulediameter, tubule wall thickness and other structural elements that areimportant in achieving desired material properties that may required forspecific applications. Such drawbacks are limiting factors that restrictthe widespread use of CNTs in potential applications.

Most of the prior art methods provide methods for synthesis of linearCNTs without morphology control. However, the anomalous electricalproperties exhibited by “kinked” or bent CNT tubules is indicative ofthe importance of non-linear, branched tubule structures in thedevelopment of CNT based electronic devices such as micro-transistorsand nanocircuits. Although it is theoretically possible to introduce awide range of structural defects with useful electronic properties inCNTs, synthetic limitations have precluded such introduction ofsystematic structural defects. Furthermore, currently available methodsdo not allow controlled alteration of linear tubule structures duringtheir growth. Post growth modifications of CNTs have been difficult toimplement and are prone to uncontrolled and random defects. Li et aldisclose a method to synthesize a CNT with a branched Y-junction(Nature, (1999) Vol. 402, 253-4) that involves the deposition of carbononto an thin aluminum sheet wherein Y-shaped molds are etched by anelectrochemical process. The CNTs formed within the aluminum molds arethen removed from within the said molds. The branched Y-shaped CNTsobtained by the method however, are not symmetrical with respect to armlength, straightness and angles between arms, since their shape andsymmetry is determined by limitations in fabrication of the aluminummold in which they are formed. Such processes are also not suited forlarge scale manufacture of CNTs and are, therefore, not economicallyviable for use in a commercial process.

SUMMARY OF THE INVENTION

The present invention provides CNTs with novel structural andmorphological characteristics, catalyst materials for CNT synthesis andmethods for manufacturing non-aligned CNTs having varied morphologies inrelatively high purity wherein tubule morphology, yield and purity areall controllable by choice of optimal process parameters. Further, thepresent invention provides apparatus and methods for the manufacture ofnon-aligned CNTs with specific morphologies in relatively high purity inhigh yields, even at atmospheric pressure. Kilogram quantities of CNTproduct can be synthesized by utilization of the methods of the presentinvention.

The present invention provides linear and branched CNTs, particularlynon-aligned CNTs, with different tubule morphologies that include, forexample, (1) cylindrical hollow single-walled and multi-walled nanotubestructures (SWNT and MWNT respectively), (2) conically overlapping or“bamboo-like” tubule structures, wherein successive end-capped graphenelayers comprising individual tubules are staggered in a telescoping,stacked arrangement; and (3) branched or “Y-shaped” tubule structureswith symmetric, straight-armed tubules forming fixed angles betweenindividual arms. Linear CNTs as defined herein, refers to CNTs that donot contain any branches originating from the surface of individual CNTtubules along their linear axes. Branched CNTs as defined herein, refersto non-linear CNTs with at least one location along the linear tubuleaxis or at the tubule terminal from which one or more tubules originate,having linear tubule axes that are non-identical to the tubule fromwhich they originate. Such points of origination of additional tubules(branch points) are also referred to herein as “junctions.” BranchedCNTs can include, for example, “Y-shaped” CNTs and “kinked” CNTs. Themethods of the present invention allow the control of morphology andstructural characteristics of individual CNT tubules during theirformation, thereby enabling the synthesis of CNTs with specificmorphology, structure, mechanical and chemical properties. Thus CNTshaving either a cylindrical, hollow tubule structure with concentricgraphene layers, or a conical “bamboo-like” structure wherein successiveend-capped graphene layers are staggered in a telescoping, stackedarrangement can be produced by the methods of the present invention.These configurations are schematically illustrated in FIG. 1.

The present invention also provides catalyst materials useful for thesynthesis of CNTs of pre-determined morphologies and methods forutilizing them in the manufacture of varied morphology CNTs. Thecatalyst materials of the invention are contacted with a carbon sourcegas either by itself or in combination with a promoter gas at anelevated temperature within the confines of a reaction chamber. Thecatalyst materials of the present invention are comprised of a substratethat includes a metallic catalyst (such as for example, a transitionmetal) hereinafter referred to as “catalytic substrate”, a catalyst gasor “promoter gas” that is capable of promoting the activity of thecatalytic substrate resulting in increased yield of the CNT products, ora combination thereof. For synthesis of linear CNTs, the catalystmaterial comprises a promoter gas and a catalytic substrate. Forsynthesis of branched CNTs, the catalyst material can comprise thecatalytic substrate either by itself, or a combination of the catalyticsubstrate with the promoter gas. The substrate for the synthesis ofsymmetrical, branched (e.g., Y-shaped) CNTs comprises a transition metalthat is supported on a metallic material or a non-metallic material,such as for example, a non-metallic oxide. The catalytic substrate canbe either distributed within or deposited on the interior surface of thereaction chamber of the manufacturing apparatus of the invention. In oneembodiment, the catalytic substrate is distributed on the surface of areaction vessel as a thin layer. The reaction vessel containing thecatalyst layer is then placed in a pyrolytic reaction chamber of areactor apparatus comprising a heater assembly that enables depositionof carbon by pyrolysis of the carbon source gas at an elevatedtemperature. The carbon source gas in the methods of the presentinvention can additionally contain a promoter gas that enables rapidcarbon deposition and CNT growth on the surface of the catalyticsubstrate.

Control of CNT morphology and tubule structure can be accomplished byvarying the parameters of the manufacturing process described by themethods of the present invention, for example, by varying the pressureof carbon source/promoter gas mixture within the CVD reaction chamber,and by varying the reaction temperature, respectively. The pressure ofthe carbon source gas/promoter gas mixture may be low (about 0.001 toabout 200 torr), moderate (about 200 to about 400 torr) or high (about400 to about 760 torr). CNT morphology can be varied during theirformation by the methods of the present invention depending on the ofgas pressure range selected, whereby either a single morphology type ora mixture of morphologies may be selectively obtained. At low gaspressures, CNTs with a cylindrical, hollow tubule morphology isobtained, whereas higher gas pressures yield parabolically shaped(conical), telescoping stacked tubule (“bamboo-like”) morphology CNTswith capped ends; CNTs with mixed tubule morphologies is attainable at amoderate pressure range. The methods of the invention also allow thecontrol of tubule diameter, tubule length, number of concentric graphenelayers (graphitization) comprising individual tubules and the yield theCNT products by variably controlling the reaction temperature of CNTsynthetic process. The reaction temperature range in the methods of theinvention ranges between about 600° and about 1500° C., preferablybetween about 700° C. and about 1200° C. In a preferred embodiment, thereaction temperature ranges between about 750° C. and about 900° C.Important structural attributes of CNTs that determine their mechanicaland electrical properties can, therefore, be controlled and “tailored”for application specific requirements utilizing the methods of theinvention. CNTs of pre-determined morphology and structural attributesare obtained in high yields (up to 700% based on catalyst substrateweight) by the methods of the invention. As will become evident from theembodiments and examples described herein, CNTs with preciselycontrolled morphology and structure can be manufactured by controllingthe optimal process parameters in the methods of the invention.

In one aspect, the catalyst materials of the present invention comprisea particulate or micro-particulate, mesoporous catalyst substrate thatcan be used for synthesis of linear and branched CNTs. The catalyticsubstrate preferably is distributed on the surface of a reaction vesselas a thin layer, following which the reaction vessel containing thecatalyst layer is placed in a reaction chamber of a reactor apparatus,such as for example, a chemical vapor deposition (CVD) reactor,comprising a heater assembly to enable chemical vapor deposition ofcarbon from a carbon source gas. In accordance with the methods of theinvention, the carbon source gas is mixed with a promoter gas in thepresence of the catalytic substrate at an elevated temperature withinthe reaction chamber. The promoter gas enables rapid chemical vapordeposition and graphitization of carbon on the catalytic substrateresulting in tubule growth on the surface and within the pores of thecatalytic substrate.

An important aspect of the present invention is the preparation andutilization of different types of catalyst materials for obtaining CNTsof specific morphologies. For synthesis of linear CNTs, the catalystmaterial comprises a particulate or micro-particulate mesoporouscatalytic substrate in combination with a promoter gas in a CVD processthat provides CNT growth initiation and facilitates rapid CNT tubulegrowth at reaction temperatures of about 600° C., which is substantiallylower than typical initiation temperatures by conventional methods(≧700° C.) in relatively high yield (up to about 700% based on catalystweight). For synthesis of branched Y-shaped CNTs, the catalyst substratecomprises a catalytic metal composed of at least one transition metalthat is supported on a metallic or non-metallic material (for example, anon-metallic oxide), which may be in particulate or micro-particulateform. The catalytic metal can be deposited on the surface of themetallic or non-metallic material as a coating on the supportingmaterial. A promoter gas or gas mixture component additionally can beintroduced in the reaction chamber during the synthesis of branched CNTsto enable rapid carbon deposition and CNT growth initiation with highgraphitization. The use of the catalyst gas in the methods of theinvention enables CNT growth on substrates that have relatively lowstrain/melting point ratios, such as for example, glass substrates usedin flat panel display (strain/melting point ≦666° C.).

The present invention provides CNTs with controlled morphology (e.g.,shape, tubule diameter, wall thickness and length, and graphitization)in relatively high yields and in large quantities (kilogram scale) thatare easily purified by a solvent wash, and methods for their preparationand manufacture. Solvents useful for purification of CNTs of the presentinvention include inorganic acids, such as for example, hydrofluoricacid (HF). The carbon nanotubes formed by methods of the presentinvention have several applications. They can be used as an additive toprovide improved strength and reinforcement to plastics, rubber,concrete, epoxies, and other materials, using currently fiberreinforcement methods for improving material strength. Furthermore, themethods of the invention provide large quantity, cost efficientsynthetic processes for producing linear and branched CNTs that aresuited for applications in hydrogen storage devices, electrochemicalcapacitors, lithium ion batteries, high efficiency fuel cells,semiconductors, nanoelectronic components and high strength compositematerials.

The foregoing and other aspects, features and advantages of the presentinvention will become apparent from the figures, description of thedrawings and detailed description of particular embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the invention can be better understood withreference to the following detailed description and accompanyingdrawings.

FIG. 1 is a schematic drawing illustrating carbon nanotube (CNT)morphologies.

FIG. 2 shows low magnification TEM photomicrographs of CNTs grown at gaspressures of (a) 0.6 (b) 50 (c) 200 (d) 400 (e) 600 and (f) 760 torr.

FIG. 3 shows high magnification TEM photomicrographs of CNTs grown atgas pressures of (a) 0.6 (b) 200 (c) 400 and (d) 760 torr.

FIG. 4 shows SEM photomicrographs of symmetrically branched (Y-shaped)CNTs at (a) low magnification (scale bar=1 μm) and (b) highmagnification (scale bar=200 nm).

FIG. 5 shows TEM photomicrographs branched CNT Y-junctions with (a)straight hollow arms and uniform diameter (scale bar=100 nm) (b)pear-shaped particle cap at tubule terminal (scale bar=100 nm) (expandedin bottom inset) and XDS photomicrograph (top right inset) showingcomposition of particle (c) a branched CNT with a double Y-junction(scale bar=100 nm) (open tubule shown in inset) and (d) high resolutionpartial image of a well graphitized, hollow tubule Y-junction.

FIG. 6 shows a schematic illustration of the manufacturing apparatus andset up for CNT synthesis.

FIG. 7 shows schematic drawing of proposed mechanism of CNT growth atdifferent gas pressures.

FIG. 8 shows yield of CNTs (as a weight ratio of CNTs to catalystsubstrate) as a function of different gas pressures. Inset shows CNTyield with reaction time.

FIG. 9 shows SEM photomicrographs of CNTs grown at gas pressures of (a)0.6 (b) 50 (c) 200 (d) 400 (e) 600 and (f) 760 torr.

FIG. 10 a-c shows low magnification TEM photomicrographs of“bamboo-like” CNTs synthesized at (a) 800° C. (b) 950° C. and (c) 1050°C. FIG. 10 d shows CNT yield dependence on reaction temperature.

FIG. 11 shows tubule diameter dependence on compartmental density of“bamboo-like” CNTs synthesized at 800° C.

FIG. 12 shows high-resolution TEM photomicrographs of “bamboo-like” CNTssynthesized at (a) 650° C. (b) 800° C. and (c) 1050° C.

DETAILED DESCRIPTION OF THE INVENTION

In one aspect, the present invention relates to linear and non-linearCNTs with specific morphological characteristics and methods forproducing them. The morphology of individual CNT tubules of can beconfigured to be linear and cylindrical with a hollow core, or stacked,conical segments (“bamboo-like”) with capped ends. In another aspect,the present invention relates to catalyst materials that are useful inmanufacturing CNTs of varied morphology that are substantially free fromdefects in relatively high purity and in high yields. The presentinvention provides methods for preparation of such catalyst materialsand their use in CNT synthetic processes. In yet another aspect, thepresent invention relates to apparatus and methods for the preparationand of varied morphology CNTs in high yields that are both conducive tocommercial manufacturability and economically viable. The catalystmaterials and CNT preparation methods of the present invention alsoallow the synthesis of CNTs with mixed morphologies by varying theappropriate reaction process parameters that are described herein.

In one embodiment, the materials containing the linear CNTs of thepresent invention comprise a plurality of free standing, linearlyextending carbon nanotubes originating from and attached to the surfaceof a catalytic substrate having a micro-particulate, mesoporousstructure with particle size ranging from about 0.1 μm to about 100 μm,and extending outwardly from the substrate outer surface. The morphologyof individual CNT tubules is either cylindrical with a hollow core, oris end-capped, stacked and conical (“bamboo-like”). Both morphologicalforms may be comprised of either a single layer or multiple layers ofgraphitized carbon. In another embodiment, the CNTs of the presentinvention are separated from the catalytic substrate material and existin a free-standing, unsupported form.

The structural attributes of the varied morphology CNTs of the presentinvention, the apparatus and methods for their synthesis andmanufacture, and the preparation and use of catalyst materials useful insuch manufacturing process is described herein with reference to thefigures and diagrams.

Referring to FIG. 1, linear CNTs of the present invention arenon-aligned, substantially linear, concentric tubules with hollow cores,or capped conical tubules stacked in a bamboo-like arrangement. Thenanotube morphology can be controlled by choosing an appropriatecatalyst material and reaction conditions in the synthetic methods ofthe present invention as detailed in different embodiments describedherein. Depending on choice of reaction conditions, relatively largequantities (kilogram scale) of morphologically controlled CNTssubstantially free of impurity related defects, such as for example,from entrapment of amorphous carbon or catalyst particles, can beobtained. The linear CNTs obtained by the methods of the presentinvention have diameters ranging from about 0.7 to about 200 nanometers(nm) and are comprised of a single or a plurality of concentric graphenelayers (graphitized carbon). The nanotube diameter and graphene layerarrangement may be controlled by optimization of reaction temperatureduring their synthesis. Thus the method of the present invention allowsthe synthesis of carbon nanotubes with defined tubule diameters anddesired layered structure.

Referring to FIG. 2, low magnification TEM images of linear CNTs grownat low, intermediate and high gas pressures are indicative that tubulemorphology can be controllably changed by choice of gas pressure“feeding” into a reactor for CNT preparation. The control of gaspressures in the methods of the present invention is accomplished byregulating gas pressure of the gases feeding in to the reactor usingconventional pressure regulator devices. CNTs grown at a gas pressure of0.6 torr (FIG. 2 a) predominantly have a morphology that consists of atubular configuration, completely hollow cores, small diameter, and asmooth surface. At 50 torr, morphology is essentially similar to that at0.6 torr, except that a small amount of tubules have an end cappedconically shaped stacked configuration (“bamboo-like”) (FIG. 2 b). At agas pressure of 200 torr, the morphology of the CNTs are predominantlythe end-capped, conical stacked configuration (“bamboo-like”) regardlessof their outer diameters and wall thickness. The density of thecompartments within individual tubules of the CNTs is high, withinter-compartmental distance inside these “bamboo-like” structuresranging from 25 to 80 nm (FIG. 2 c). At gas pressures greater than 200torr, an entirely “bamboo-like” morphology is obtained for the CNTs,with increased compartmental density. The inter-compartmental distanceswithin the individual CNTs decrease with increasing gas pressure (10-50nm at 400 torr and 10-40 nm at 600 and 760 torr respectively). CNTssynthesized at 760 torr have a wider tubule diameter (20 nm to 55 nm(FIG. 2 f), have thin walls and smooth surfaces. In comparison to linearCNTs synthesized at a gas pressure of 200 torr, those synthesized athigher pressures (400 and 600 torr) are highly curved and have brokenends (FIGS. 2 d and 2 e). This is attributed to fracturing of the CNTsduring the TEM specimen preparation, which is indicative that CNTs witha “bamboo-like” morphology may be readily cleaved into shorter sectionscompared to the tubular type.

CNTs of the present invention have a relatively high degree ofgraphitization (process of forming a planar graphite structure or“graphene” layer). The complete formation of crystalline graphenelayers, and the formation of multiple concentric layers within eachtubule and hollow core, with minimal defects (such as those typicallycaused by entrapment of non-graphitized, amorphous carbon and metalcatalyst particles) is an important prerequisite for superior mechanicalproperties in CNTs. Referring to FIG. 3 which shows TEM photomicrographsdetailing morphologies of linear CNTs grown at different gas pressures,it is seen that CNTs grown at pressures between 0.6 to 200 torr havegood graphitization, their walls comprising of about 10 graphene layersall of which terminate at the end of the CNT that is distal from thesubstrate (i.e. the fringes are parallel to the axis of the CNT), andpossess completely hollow cores. Linear CNTs grown at 200 torr havetubule walls comprising about 15 graphene layers. Individual tubules are“bamboo-like” rather than completely hollow, with diaphragms thatcontain low number (≦5) of graphene layers. All graphene layersterminate at the surface of the CNTs, with the angle between the fringesof the wall and the axis of the CNT (the inclination angle) being about3° (FIG. 3 b). Linear CNTs grown at intermediate gas pressures (400-600torr) have a “bamboo-like” structure, more of graphene layers in thewalls and diaphragms of tubules (typically 25 and 10 graphene layers intheir walls and diaphragms, respectively (FIG. 3 c), but lessgraphitization (lower crystallinity) due to a faster growth rate.Despite the low crystallinity, all graphene layers terminate on thetubule surface with inclination angle of about 6°. CNTs grown at 760torr have higher graphitization than those grown at 400-600 torr, have a“bamboo-like” structural morphology consisting of parabolic-shapedlayers stacked regularly along their symmetric axes (FIG. 3 d). All thegraphene layers terminate within a short length along growth directionon the surface of the CNTs resulting in a high density of exposed edgesfor individual layers. The inclination angle of the fringes on the wallof the CNTs is about 13° (FIG. 3 d). The high number of terminal carbonatoms on the tubule surface is expected to impart differentiatedchemical and mechanical properties in these CNTs compared the hollow,tubular type, and render them more amenable for attachment of organicmolecules.

In another aspect, the present invention relates CNTs comprising abranched (“Y-shaped”) morphology, referred to herein as “branched CNTs”,wherein the individual arms constituting branched tubules are eithersymmetrical or unsymmetrical with respect to both arm lengths and theangle between adjacent arms. In one embodiment, the Y-shaped CNTs existas (1) a plurality of free standing, branched CNTs attached to thesubstrate and extending outwardly from the substrate outer surface; and(2) one or more CNTs with a branched morphology wherein the CNT tubulestructures have Y-junctions with substantially straight tubular arms andsubstantially fixed angles between said arms.

As seen in FIG. 4, branched CNTs synthesized by the methods of theinvention comprise a plurality of Y-junctions with substantiallystraight arms extending linearly from said junctions. The majority ofbranched CNTs possess Y-junctions having two long arms that are a fewmicrons long (about 2 to about 10 μm), and a third arm that is shorter(about 0.01 to 2 μm). CNTs with Y-junctions comprising three long arms(up to about 10 μM), and with multiple branching forming multipleY-junctions with substantially linear, straight arms can be alsoobtained by the method of the invention. High magnification SEMmicrograph (FIG. 4 b) shows that the branched CNTs of the inventionpossess Y-junctions that have a smooth surface and uniform tubulediameter about 2000 nm. The angles between adjacent arms are close toabout 120°, thereby resulting in branched CNTs that have a substantiallysymmetric structure. All Y-junctions have a substantially similarstructural configuration, regardless of their varying tubule diameters.

Referring to FIG. 5, it is seen that most Y-junctions of branched CNTsof the present invention have hollow cores within their tubular arms(FIG. 5 a). A triangular, amorphous particle is frequently found at thecenter of the Y-junction (inset in FIG. 5 a). Compositional analysis byan energy dispersive x-ray spectrometry (EDS) indicates that thetriangular particles consist of calcium (Ca), silicon (Si), magnesium(Mg), and oxygen (O). The Ca and Si are probably initially contained inthe catalyst material. It is frequently observed that one of the twolong arms of the Y-junction is capped with a pear-shaped particle (FIG.5 b and lower inset), that a similar chemical composition as that of theaforementioned triangle-shaped particle found within the tubules at theY-junction. A trace amount of cobalt (Co) from the catalytic material isfound at the surface of such pear-shaped particle. The tubule of theother long arm of the branched CNT (FIG. 5 b) is filled with crystallinemagnesium oxide (MgO) from the catalytic material (confirmed bydiffraction contrast image in the EDS spectrograph). Selected areadiffraction patterns (upper right inset in FIG. 5 b) indicate that_oneof <110> reflections, [101], of the MgO rod is parallel to [0002]reflection (indicated by arrow heads) from carbon nanotube walls.Therefore, MgO rod axis is along [210]. Additionally, Y-junctions filledwith continuous single crystalline MgO from one arm, across a joint, toanother arm can also be obtained. FIG. 12 c shows a double Y-junction,wherein only one arm of the right-side Y-junction is filled with singlecrystal MgO. FIG. 5 b (inset) shows a magnified image of the end of theMgO filled arm, illustrating an open tip that provides entry of MgO intothe CNT Y-junctions. FIG. 5 d shows a highly magnified partialY-junction, which is well graphitized, and consists of about 60concentric graphite layers (partially shown) in its tubule arms, and ahollow core with a diameter of about 8.5 nm.

Linear CNTs produced according to the present invention can be utilizedto form composites with other materials, especially dissimilarmaterials. Suitable dissimilar materials include metals, ceramics,glasses, polymers, graphite, and mixtures thereof. Such composites maybe prepared, for example, by coating the products of the presentinvention with these dissimilar materials either in a solid particulateform or in a liquid form. A variety of polymers, includingthermoplastics and resins can be utilized to form composites with theproducts of the present invention. Such polymers include, for examplepolyamides, polyesters, polyethers, polyphenylenes, polysulfones,polyurethanes or epoxy resins. Branched CNTs of the present inventioncan find application in construction of nanoelectronic devices and infiber-reinforced composites. The Y-junction CNT fibers of the inventionare expected to provide superior reinforcement to composites compared tolinear CNTs.

The present invention also provides catalyst materials for synthesis ofmorphologically controllable CNTs that comprise a catalyst substrate anda promoter gas that is used in combination with a carbon source gas. Thecatalytic substrate component can be used in the CNT synthetic processeither by itself to cause reaction of the carbon source gas, or incombination with the promoter gas that is mixed with the carbon sourcegas. The catalyst substrate component of the present invention compriseseither a single metallic material or combination of metals, a metallicalloy, a metal/metallic alloy combination, organometallic compounds, orcombinations thereof that is impregnated with or deposited on thesurface and within the pores of a particulate or micro-particulate,mesoporous sol-gel. The micro-particulate sol-gel can be, for example,mesoporous silica, mesoporous alumina or mixtures and combinationsthereof. A combination of sol-gels may also be used as the catalystsubstrate component.

In a preferred embodiment, the metallic material is impregnated into thesol-gel as nanoparticles. Particle sizes of the catalyst substratematerials range between about 0.01 and about 100 microns (μM). Due totheir particulate and porous nature, the catalyst substrate materials ofthe present invention provide a catalyst surface area (relative to theof weight of catalyst substrate) that is substantially higher thantraditional catalysts used for CNT synthesis, and therefore, providesubstantially higher yields of CNT product. The catalytic substratesconstituting the catalyst materials of the present invention forproducing branched CNTs preferably comprise at least one transitionmetal or metal alloy that is deposited on or impregnated within asupport substrate comprising a metallic material or a non-metallicmaterial, such as for example, a non-metallic oxide. The metallicmaterial or non-metallic oxide comprising the catalytic substrate can beeither a transition or a non-transition metallic oxide, or anon-metallic inorganic oxide. Preferred metallic materials in thecatalytic substrates of the invention include iron, cobalt, nickel, oralloys and combinations thereof. Preferred transition metals includeiron, cobalt and nickel. In a currently preferred embodiment, thetransition metal is cobalt. Metallic oxides useful in the catalystmaterials include, for example, oxides of beryllium, magnesium, calcium,strontium and barium. Preferred metallic oxides include magnesium oxideand calcium oxide. In a currently preferred embodiment, the metallicoxide is magnesium oxide (MgO).

Preferred organometallic compounds are metallocenes, such as forexample, ferrocene, nickelocene, a ferrocene-xylene mixture, orcombinations thereof. The organometallic compounds can be additionallycombined with an organic compound, such as for example, a combination ofnickelocene and thiophene. The metallic oxide support substrate of theinvention can be either in fused pieces or in a particulate form.Preferred forms for the metallic oxide support includes fused pieces orfused particles (average size from about 0.1 to about 1000 μm). In acurrently preferred embodiment, the catalytic substrate comprises ironnanoparticles that are impregnated in mesoporous silica, which is groundinto micro-particles to increase reactive surface area of the catalystsubstrate. In a preferred embodiment, the particle size of the catalystsubstrate ranges from about 0.1 to about 100 μm. In a currentembodiment, the metallic oxide support substrate has an average particlesize of about 50 μM.

In a one embodiment, the catalytic substrate of the invention forproducing branched CNTs is prepared by immersing metallic oxideparticles, such as for example,_magnesium oxide, in an alcoholic metalsalt solution, such as for example cobalt nitrate, under conditionssufficient to cause the cobalt to become impregnated into the metallicoxide. The magnesium oxide with impregnated cobalt is filtered, driedand calcined at elevated temperature (preferably about 130° C.) for anextended period of time (preferably about 14 hours).

The promoter gas component of the catalyst materials of the presentinvention can be a substance that is a gaseous compound at the reactiontemperatures, and preferably comprises a non-carbon gas such as ammonia,ammonia-nitrogen, hydrogen, thiophene, or mixtures thereof. The promotergas of the present invention may be diluted by mixing it with a diluentgas, which are primarily unreactive, oxygen-free gases, such as forexample, hydrogen, helium, nitrogen, argon, neon, krypton, xenon,hydrogen sulfide, or combinations thereof. For the CVD reaction processof the present invention, hydrogen is preferred for reactiontemperatures maintained at less than about 700° C., while for highertemperatures (≧700° C.), the promoter gas is chosen from ammonia,hydrogen, nitrogen, or any combination thereof. The promoter gas can beintroduced into the reaction chamber of the reaction apparatus (e.g. theCVD reaction chamber) at any stage of the reaction process. Preferably,the promoter gas is introduced into the reaction chamber either prior toor simultaneously with the carbon source gas. The CNT nanotubenucleation process on the catalyst substrate is catalyzed by thepromoter gas of the present invention enables every metal catalyst “cap”that is formed within individual tubules to catalyze their efficient andrapid growth.

The carbon source gas of the present invention can be saturated,unsaturated linear branched or cyclic hydrocarbons, or mixtures thereof,that are in either in the gas or vapor phase at the temperatures atwhich they are contacted with the catalytic substrate material (reactiontemperature). Preferred carbon source gases include methane, propane,acetylene, ethylene, benzene, or mixtures thereof. In a currentlypreferred embodiment, the carbon source gas for the synthesis of linearCNTs is acetylene, and for the synthesis of branched CNTs, the carbonsource gas is methane.

Production of linear CNT materials of the present invention isaccomplished by distribution of micro-particulate catalyst substrate onthe surface of an open container (boat), which is then placed into thereaction chamber of a CVD apparatus and exposed to a flow of a gasmixture containing the carbon source gas and a promoter gas. Thereaction temperature, gas pressure and reaction time are maintainedunder pre-determined conditions effective to cause formation and growthof a plurality of carbon nanotubes on the catalyst substrate surface.The CVD chamber temperature and gas pressure are optimized to controland obtain the desired the morphology of carbon nanotubes during theirgrowth.

FIG. 6 shows a schematic illustration of one embodiment of amanufacturing apparatus and assembly for CNT production by the method ofthe invention by a batch process. A reaction chamber 1 has an internalvolume capable of accommodating sample boat 2 that is capable ofcontaining the catalyst material for CNT growth. Chamber 2 comprises aheater 3 controlled by a controller 4 that enables the reactiontemperature inside chamber 2 to be maintained at a level so as to enableinitiation of CNT growth. The flow rate of the carbon source gas andpromoter gas mixture 6 into chamber 2 is controlled by a mass flowcontroller 5. Pressure transducer 7 enables monitoring of gas pressureinside chamber 2 which is controlled by valve 9 that is operated byvalve controller 8. Vacuum pump 10 is capable of evacuating andmaintaining the appropriate pressure inside chamber 2. The manufacturingapparatus and assembly of the present invention can be readilyconfigured by standard methods known in the art to function in acontinues process for the scaled up synthesis of CNTs with controlledmorphology.

In a one embodiment, linear CNTs of the present invention aresynthesized by the CVD deposition of a carbon source gas in a reactionchamber in the presence of a promoter gas on a metal impregnated,micro-particulate mesoporous silica sol-gel catalyst, while maintainingan optimum reaction temperature range (between about 600° and about1500° C.). The volume ratio of carbon source gas to promoter gas ismaintained between 1:2 to 1:10 and optimum gas pressure is preferablymaintained between about 0.1 to about 760 torr.

In another embodiment, branched CNTs of the invention are synthesized inhigh purity and yield by pyrolysis of a carbon source gas at elevatedtemperature in the presence of the catalyst material. In a currentlypreferred embodiment, branched CNTs are synthesized by pyrolysis of acarbon source gas, such as for example methane, on a catalyst substratecomprising a transition metal deposited on a metal oxide at atemperature of about 1000° C. for about 1 hour. A promoter gas such asfor example, ammonia, hydrogen, nitrogen, thiophene or mixtures thereofis additionally introduced into the reaction chamber to provide rapidgrowth of CNTs. The carbon source gas flow is maintained between about 1sccm and about 1000 sccm (standard cubic centimeter/minute), and thereaction temperature is maintained between about 600° C. and about 1500°C.

The manufacturing methods of the invention also enable the tailoring oflinear CNT morphology by controlling gas pressure. At relatively lowpressures, CNTs with a tubular hollow structure can be obtained, whereasat relatively high pressures, CNTs with “bamboo-like” structure andincreased compartmental density can be obtained. The number of graphenelayers, which is related to thickness of the tubule wall and diaphragmof the CNTs, can also be controlled during their formation by control ofgas pressure. Once the first layer forms as a bamboo-like structure, allsubsequent layers terminate on the surface of the CNT.

The methods of the present invention allow the process parameters forCNT formation to be varied optimally, thereby enabling controllableformation of CNTs with pre-determined morphologies. The growth mechanismfor linear CNTs obtained by the methods of the invention likely involvesthe outward growth from the both the surface and from within the poresof the mesoporous catalyst substrate. Carbon atoms from the carbon gassource dissolve into catalyst droplets, diffuse through the catalyticparticle and precipitate on the other side of the catalyst droplet toform CNTs. The effect of growth rate and CNT tubule morphology on gaspressure affects the two different carbon atom precipitation areas (aand b) on the mesoporous catalyst substrate comprising iron (Fe)particles are shown in FIG. 7. Since the carbon source gas substantiallydecomposes into carbon atoms involved in CNT tubule growth, such avariation of reaction gas pressure results in a change in carbon atomconcentration, which, in turn, influences the manner in which CNTtubules grow. At low pressure, when concentration of the carbon atoms islow and both dissolution and diffusion are limited, carbon atomsdissolved into the catalyst particles prefer to diffuse towards area arather than the distant, opposite area b. The carbon atoms precipitatedin area b form completely hollow CNTs. Due to the low concentration ofthe carbon atoms, the growth rate of CNTs is suppressed, so the yield isrelatively low. At high reaction gas pressure, the concentration ofcarbon atoms increases, the dissolution rate of carbon atoms into thecatalyst particles increases, resulting in an increase in theconcentration of carbon atoms dissolved inside the catalyst particles.The enriched carbon atoms can diffuse towards both areas a and b on thecatalyst particles, then the carbon atoms precipitate to form dome-likecarbon shell (FIG. 7 c); for simplification only one graphene layer isshown). However, due to the low concentration of carbon atoms and thesteric characteristic of the catalyst particles the diffuse ratestowards area a is greater than that towards area b. The precipitationrates of carbon atoms at area a is therefore, faster than in area b.Consequently, a concentric graphene layer with closed cap and definedlength forms (FIG. 7 d). A second dome-like carbon shell is formedsubsequently in a similar manner. This process is repetitive and resultsin multiple analogous capped graphene layers that are separated by afixed distance along the direction of tubule growth (growth axis),resulting in a “bamboo-like” CNT with uniform density of compartments(FIG. 7 e). The concentric graphene layers have a relatively largerdiameter at the growth ends than at the capped ends. While the diametersat the capped ends are fixed after their formation, the diameters at theopen growth ends continue to increase with continued tubule growth. Oncethe larger open end diameter of a graphene layer exceeds the carbonprecipitation area a on the catalyst particle, the layer stops growingdue to non-availability of carbon atoms, and leaves its edge on thesurface of CNTs (FIG. 7 f). As this process continues, all the initialinner layers shift out, in turn, to the surface then terminate in aregular manner. At high carbon atom concentrations obtained at highreaction gas pressure, the dissolution, diffusion and precipitationrates of carbon atoms increase substantially. As a result, a pluralityof graphene layers precipitate to form a multi-layered diaphragm in the“bamboo-like” CNT. CNTs grown at a high precipitation rate of carbonatoms therefore, have high compartmental density, which is indicative ofthe graphene layers being shifted out to the surface at a high rate. Asa result, the graphene layer edges exposed on the surface of the CNTsalso have a high density and a high inclination angle with reference tothe CNT axis.

The manufacturing methods of the present invention provide high yieldsof linear CNTs relative to current methods; yields of up to about 700%based on weight of catalyst substrate can be obtained by choice ofoptimal process parameters provided by the methods of the presentinvention as described herein. In one embodiment of the invention, theyield (weight ratio of CNTs to the catalyst substrate) of CNTs isoptimized by maintaining optimum gas pressure and reaction temperaturein the chamber during the growth. In a currently preferred embodiment,the catalyst substrate is iron embedded mesoporous silica. The effect ofcontrolling gas pressure on CNT yield in the methods of the invention isshown in FIG. 8. The relative yield (based on weight of catalystsubstrate) is lower at low and high gas pressures (about 140% and about350% at around 0.6 torr and 760 torr, respectively), and issubstantially higher at intermediate pressures (over 600% at around 600torr). The methods of the present invention further provide the abilityto increase CNT yield by selecting an optimal reaction time for CNTformation at a pre-selected optimal gas pressure. As shown in FIG. 6(inset), the CNT yield increases substantially with an increasedreaction time; for example, compared to the CNT yield after a reactiontime of 2 hours, the relative yield increases by about 25% at around 5hours and over 40% at about 12 hours. The optimal relative yield of CNTscan therefore, be maximized at for a selected gas pressure (for example,a relative CNT yield of about 200% can be obtained for a reaction timeof 12 hours at 0.6 torr) by the methods of the present invention.

The methods of the present invention further provides the ability tocontrol CNT morphology, including individual tubule diameter size andsurface roughness, particularly for linear CNTs during their formation.FIG. 9 shows scanning electron microscopy (SEM) photomicrographs of theCNTs grown at gas pressures ranging from about 0.6 torr to about 760torr. CNTs grown at relatively low gas pressures of about 0.6 torr toabout 50 torr have relatively smaller individual tubules with diametersthat are distributed over a narrow range, smooth surfaces and a tubulelength of about 10 to 20 μm (FIGS. 9 a and 9 b). At relatively moderategas pressures (about 200 torr), a fraction of CNTs tubules withrelatively larger, non-uniform diameters with smooth surfaces areobserved (FIG. 7 c). In contrast, CNTs grown at relatively higher gaspressures (about 400 torr to about 600 torr) have larger uniform tubulediameters, increased surface roughness, exhibit higher degrees ofcurvature and have relatively shorter tubule lengths (FIGS. 7 d and 7e). At gas pressures above around 760 torr, the CNT tubule diameters arenon-uniform (FIG. 7 f) and bear similarity to those obtained around 200torr; tubule surfaces are however, smooth. When observed under SEM underidentical conditions, photomicrographs obtained for CNTs synthesizedbelow 200 torr are much clearer than those synthesized above 400 torr.This observation, combined with the difference in morphologies, isindicative of micro-structural differences between these CNTs obtainedat different pressures. SEM photomicrographs further indicate that CNTssynthesized over the entire range of pressures by methods of theinvention have a relatively high degree of purity.

The methods of the present invention provide yet another way ofcontrolling CNT yield (particularly for linear CNTs) by optimallyadjusting reaction temperature during CNT formation. Table 1 shows thedependence CNT yield on reaction (growth) temperature. CNT yield by themethods of the invention is highest at intermediate reactiontemperatures (reaching about 700 weight % relative to weight ofsubstrate catalyst). The yields listed in Table 1 are for CNTs in a CVDreaction comprising a microparticulate catalyst substrate that isdistributed as a thick layer (mechanical spreading of about 100 mgmicro-particulate catalyst substrate on the surface of a rectangularsample boat). Since CNT yields by the methods of the invention isdirectly dependent on catalyst surface area, higher yields can beobtained by spreading the catalyst particle layer over a larger areawithin the reaction chamber.

TABLE 1 Temperature dependence on yield of CNTs obtained by CVD processReaction 650 700 750 800 900 950 1000 1050 temperature (° C.) CNT yield30 177 388 378 689 644 481 235 (weight %)* *Relative yield based onweight of substrate catalyst

The methods of the present invention allow the control of CNT morphologyand individual tubule structure, particularly for linear CNTs bycontrolling the reaction temperature during CNT formation. Based on thelow magnification TEM photomicrographs of the CNTs grown at varioustemperatures (FIG. 10) by the methods of the present invention, it isseen that all the CNTs have a “bamboo-like” structure. The interiors ofCNT tubules show that the structure is identical along their length,with the compartments being spaced in an almost equidistant manner. Forexample, the density of the compartments per about 100 nm along thetubule axial length was measured from the TEM photomicrographs of CNTsobtained at a reaction temperature of 800° C. (FIG. 11). Thecompartmental density decreases with the increasing of diameters of theCNTs, and is dependent on the CNT growth temperatures. The tubulediameters of CNTs vary over a relatively wide range at as a function ofCNT growth temperatures. CNTs tubules grown in the range spanning 650°C., 750° C., 800° C., 950° C. and 1050° C. have diameters ranging fromabout 10 to about 20 nm, about 20 to about 54 nm, about 20 to about 69nm, about 20 to about 88 nm, and about 60 to about 186 nm, respectively.Although the overall distribution of CNT tubule diameter increases withthe increase in CNT growth temperature, a majority of tubules havediameters that are distributed over a relatively narrow range,particularly at higher reaction temperatures. Since the tubule diametersof the CNTs are primarily determined by the size of catalyst particlesinvolved in the reaction process, such as for example, a CVD process,the increase of CNT diameter with increasing reaction temperature isindicative of larger catalyst particles being formed at higher growthtemperatures.

Besides control of CNT tubule diameter, the methods of the presentinvention allows for the control of graphitization of individual tubulesand the number of concentric graphene layers comprised in the CNTs canbe controlled by controlling reaction CNT growth temperatures in themethods of the present invention. FIG. 12 shows high magnification TEMimages of CNTs grown at varied temperatures. With increasing growthtemperatures, for example at about 650° C., 800° C., and 1050° C., theCNT tubule outer diameters are approximately 13, 23, and 65 nm,respectively. The number of graphene layers contained within the wallsat these respective growth temperatures are approximately 12, 24, and50, respectively, while the graphene layers contained in theirdiaphragms are approximately 3, 9, and 20, respectively. Unlike CNTswith morphology comprising concentric cylindrical layers in their walls,CNTs with “bamboo-like” morphology have parabolic shaped layers stackedregularly along the length. The graphene layers of the bamboo-like CNTsterminate at the surface of the tubes, whereby the edges of the graphenelayers are exposed. Both the number of graphene layers within the CNTtubule wall and the diaphragm of the compartment, therefore, increasewith the temperature, while graphitization of the CNT tubules issignificantly improved with increase in reaction temperature in methodsof the present invention.

The synthetic methods for preparation of compounds and materials of thepresent invention, and examples of CNTs, catalytic materials andcatalytic substrates are described in the following examples, which arenot intended to be limiting in any manner with regards to the scope ofthe invention.

EXAMPLES Example 1 Preparation of Catalyst Substrate for Synthesis ofLinear CNTs

Mesoporous silica containing iron nanoparticles were prepared by asol-gel process by hydrolysis of tetraethoxysilane (TEOS) in thepresence of iron nitrate in aqueous solution following the methoddescribed by Li et al. (Science, (1996), Vol. 274, 1701-3) with thefollowing modification. The catalyst gel was dried to remove excesswater and solvents and calcined for 10 hours at 450° C. and 10⁻² torr togive a silica network with substantially uniform pores containing ironoxide nanoparticles that are distributed within. The catalyst gel isthen ground into a fine, micro-particulate powder either mechanicallyusing a ball mill or manually with a pestle and mortar. The groundcatalyst particles provide particle sizes that range between 0.1 and 100μM under the grinding conditions.

Example 2 Preparation of Catalyst Substrate for Synthesis of BranchedCNTs

Magnesium oxide (MgO) supported cobalt (Co) catalysts were prepared bydissolving 0.246 g of cobalt nitrate hexahydrate (Co(NO₃)₂.6H₂O, 98%) in40 ml ethyl alcohol, following which immersing 2 g of particulate MgOpowder (−325 mesh) were added to the solution with sonication for 50minutes. The solid residue was filtered, dried and calcined at 130° C.for 14 hours.

Example 3 General Synthetic Procedure for Linear CNTs

The synthesis of CNTs is carried out in a quartz tube reactor of achemical vapor deposition (CVD) apparatus. For each synthetic run, 100mg of the micro-particulate catalyst substrate was spread onto amolybdenum boat (40×100 mm²) either mechanically with a spreader or byspraying. The reactor chamber was then evacuated to 10⁻² torr, followingwhich the temperature of the chamber was raised to 750° C. Gaseousammonia was introduced into the chamber at a flow rate of 80 sccm andmaintained for 10 minutes, following which acetylene at a flow rate of20 sccm was introduced for initiate CNT growth. The total gas pressurewithin the reaction chamber was maintained at a fixed value that rangedfrom 0.6 torr to 760 torr (depending on desired morphology for theCNTs). The reaction time was maintained constant at 2 hours for eachrun. The catalytic substrate containing attached CNTs were washed withhydrofluoric acid, dried and weighed prior to characterization.

Example 4 General Synthetic Procedure for Branched CNTs

The MgO supported cobalt catalyst of Example 3 were first reduced at1000° C. for 1 hour in a pyrolytic chamber under a flow of a mixturehydrogen (40 sccm) and nitrogen (100 sccm) at a pressure of 200 Torr.The nitrogen gas was subsequently replaced with methane (10 sccm) toinitiate CNT growth. The optimum reaction time for producing branchedCNTs was 1 hour.

Example 5 Characterization of CNT Morphology and Purity by ScanningElectron Microscopy (SEM), and Tubule Structure and Diameter byTransmission Electron Microscopy (TEM)

Scanning electron microscopy (SEM) for characterization and verificationof CNT morphology and purity was performed on a JEOL JSM-6340Fspectrophotometer that was equipped with an energy dispersive x-ray(EDS) accessory. Standard sample preparation and analytical methods wereused for the SEM characterization using a JEOL JSM-6340F microscope. SEMmicrographs of appropriate magnification were obtained to verify tubulemorphology, distribution and purity.

Transmission electron microscopy (TEM) to characterize individual tubulestructure and diameter of the CNTs was performed on a JEOL 2010 TEMmicroscope. Sample specimens for TEM analysis were prepared by mildgrinding the CNTs in anhydrous ethanol. A few drops of the groundsuspension were placed on a micro-grid covered with a perforated carbonthin film. Analysis was carried out on a JEOL 2010 microscope. TEMmicrographs of appropriate magnification were obtained for determinationof tubule structure and diameter. Although the examples described hereinhave been used to describe the present invention in detail, it isunderstood that such detail is solely for this purpose, and variationscan be made therein by those skilled in the art without departing fromthe spirit and scope of the invention.

1. A material comprising a plurality of non-aligned carbon nanotubescomprising individual tubules having a cylindrical hollow single-walledstructure.
 2. The material of claim 1 wherein the plurality ofnon-aligned carbon nanotubes are linear unbranched tubules.
 3. Thematerial of claim 2 wherein the linear unbranched tubules have a hollowcylindrical morphology having at least one graphene layer.
 4. Thematerial of claim 2 wherein the linear unbranched tubules have asegmentally conical stacked array morphology having at least onegraphene layer in each segment.
 5. The material of claim 1 wherein theplurality of non-aligned carbon nanotubes are substantially comprised ofbranched tubules having at least one branching node along a tubule axis.6. The material of claim 1 wherein the plurality of non-aligned carbonnanotubes are branched tubules with a Y-junction in at least onebranching node.
 7. The material of claim 6 wherein the angle formedbetween adjacent branch tubules forming the Y-junction is about 90° toabout 240°.
 8. The material of claim 6 wherein the angle formed betweenadjacent branch tubules forming the Y-junction is about 120°.
 9. Amaterial comprising a plurality of non-aligned carbon nanotubescomprising individual linear unbranched tubules having a cylindricalhollow multi-walled structure, wherein the linear unbranched tubuleshave a segmentally conical stacked array morphology.
 10. The material ofclaim 9 further comprising at least one graphene layer in each segmentof the conical stacked array.
 11. A catalyst substrate materialcomprising: a plurality of mesoporous sol-gel particles havingsubstantially uniform pores; and a metallic material coated on theplurality of mesoporous sol-gel particles.
 12. The catalyst substratematerial of claim 11 wherein the plurality of mesoporous sol-gelparticles range from about 10⁻³ to about 10³ microns.
 13. The catalystsubstrate material of claim 11 wherein the plurality of mesoporoussol-gel particles are mesoporous silica, mesoporous alumina or mixturesthereof.
 14. The catalyst substrate material of claim 11 wherein themetallic material is a transition metal, metal alloy or a combinationthereof.
 15. The catalyst substrate material of claim 11 wherein themetallic material is selected from the group consisting of iron, cobalt,nickel and combinations thereof.
 16. The catalyst substrate material ofclaim 11 further comprising at least one non-metallic material.
 17. Thecatalyst substrate material of claim 16 wherein the non-metallicmaterial is an organic or inorganic oxide, nitride, sulfide or carbidecompound.
 18. The catalyst substrate material of claim 16 wherein thenon-metallic material is an organo-metallic material.
 19. The catalystsubstrate material of claim 18 wherein the organo-metallic material isselected from the group consisting of ferrocene, nickelocene,cobaltocene and mixtures thereof.
 20. The catalyst substrate material ofclaim 11 wherein the metallic material is impregnated within theplurality of mesoporous sol-gel particles.